The present invention relates to an organic electroluminescent (EL) device.
More precisely, it relates to an organic EL device of which the luminance is not attenuated in long-term driving and which has good durability.
As being self-emission, EL devices have high self-distinguishability. In addition, they have high impact resistance as being completely solid devices. Therefore, the use of EL devices in various displays as light source is being widely noticed.
EL devices are grouped into inorganic EL devices in which are used inorganic compounds as light-emitting materials, and organic EL devices in which are used light-emitting organic compounds. Of those, organic EL devices have been being much studied and expected as practical light emitters in the coming generations, since they require a greatly reduced driving-voltage and they can be easily small-sized.
In the circumstances, the most important theme in practical studies of organic EL devices is to establish the technique of preventing the attenuation of the luminance of the devices in long-term driving and to provide practicable organic EL devices.
In this respect, it is known that the purity of organic compounds to be used for producing organic EL devices has a great influence on the attenuation of the luminous efficiency and the luminance of the devices produced, for example, as in xe2x80x9cMonthly DISPLAY, September, p. 15, 1995xe2x80x9d, and xe2x80x9cOYOBUTURI, Vol. 66, No. 2, pp. 114-115, 1997xe2x80x9d. However, the influences of the structures and the properties of organic compounds to be used for producing organic EL devices on the properties of the devices produced are not as yet clarified, and no method is known capable of quantitatively determine the influences in question.
Therefore, the details of the reasons why long-term use of organic EL devices brings about the attenuation of the luminance of the devices are not as yet known at present.
Given that situation, the object of the invention is to solve the problems in the prior art noted above, and to provide a practical organic EL device of which the luminance is not attenuated in long-term driving and which has good durability.
We, the present inventors have assiduously studied in order to attain the object.
Through our studies, we measured the number of electron spins existing in each organic compound to be used for forming the organic compound layers of organic EL devices, and have found that a good correlation was obtained between the number of electron spins and the properties of organic EL devices. Specifically, of many organic EL devices we produced, those which comprised organic compounds having a large number of electron spins existing therein were impracticable for long-term driving, as their luminance greatly attenuated in practical use.
The reason is because a large number of electron spins existing in organic compound layers of organic EL devices will trap the holes and electrons as injected into the layers to thereby probably increase the driving voltage for the devices and quench the excited state of the light-emitting compounds of the devices.
On the basis of this knowledge, we have found that, in order to prevent the attenuation of the luminance of organic EL devices being driven for a long period of time, at least one of organic compounds to be used for forming the organic compound layers of each organic EL device must be such that the number of electron spins existing therein is not more than 1013 per mg of the compound.
Based on this finding, we have completed the ;present invention.
Specifically, the invention provides the following:
(1) An organic EL device comprising one or more organic compound layers that include at least one organic light-emitting layer and are sandwiched between a pair of electrodes of anode and cathode, wherein at least one of organic compounds used for forming the organic compound layers is such that the number of electron spins existing therein is not more than 1013 per mg of the compound.
(2) The organic EL device of (1), wherein the organic compound layers are formed through vapor deposition.
(3) The organic EL device of (1) or (2), wherein the number of electron spins existing in the organic compound used for forming the organic light-emitting layer is not more than 1013 per mg of the compound.
(4) The organic EL device of (1) or (2), wherein the number of electron spins existing in the organic compound used for forming the organic compound layer for hole injection or transportation is not more than 1013 per mg of the compound.
(5) The organic EL device of (1) or (2), wherein the number of electron spins existing in the organic compound used for forming the organic compound layer for electron injection or transportation is not more than 1013 per mg of the compound.